This invention relates to an interference eliminator that eliminates interference signals from communication receivers and, in particular, to an interference eliminator where the center frequency and the band width of the signals passed by the receiver are continuously variable.
In conventional communication receivers, measures have been taken in order to receive signals with a predetermined intelligibility in the presence of interference caused by cross signals from numerous stations within a narrow band. For example, one of the methods for eliminating the adjacent interference signals is to provide a plurality of crystal filters, etc. with good shape factors in the receiver and to select a desired filter in order to obtain desired receiver performance. However, this method has a shortcoming in that many filters are required which increase size as well as cost.
Another means used is to equip the communication receiver with an intermediate frequency shifting circuit, which freely shifts received signals within the filter band without changing the receiver tone during SSB or CW signal reception. FIGS. 1 through 3 are circuit diagrams of intermediate frequency stages of SSB receivers equipped with intermediate frequency shifting circuits.
In FIG. 1, a mixing circuit 1, an intermediate frequency filter 2 having a bandwidth of 5.645 MHz.+-.4 KHz, a mixing circuit 3, an intermediate frequency filter 4 having a band width of 5.695 MHz.+-.1.5 KHz, a mixing circuit 5, and a notch filter 6 with a notch frequency of 50 KHz are arranged in cascade connection. Outputs are injected from an oscillator 8 into mixing circuit 1, from an oscillator 10 with a 48.5 KHz center frequency into mixing circuit 3 and a detector circuit 7, from an oscillator 9 of 5.645 MHz into mixing circuit 5 thus comprising the intermediate frequency stage of a triple conversion system. RF.sub.in indicates a high frequency signal input and AF is a low frequency output. In this circuit, the SSB signal is inputted at 5.6465 MHz.+-.1.5 KHz into the mixing circuit 3, inputted at 5.695 MHz.+-.1.5 KHz into mixing circuit 5, and inputted at 50 KHz.+-.1.5 KHz signal into notch filter 6 and detecting circuit 7.
In operation, the case will be considered where it is desired to eliminate interference waves that are approximately 1 KHz higher than the desired high frequency input signal. Since the first mixing circuit 1 utilizes a local oscillation frequency higher than the high frequency input signals, the output frequency of oscillator 10 is changed from 48.5 KHz to 48.0 KHz and the interference waves are converted to a frequency of 5.6935 MHz on the input side of intermediate frequency filter 4. The interference waves are thus eliminated by moving them below the lower end of the band width of intermediate frequency filter 4.
The circuit shown in FIG. 2 is also based on the same principle as that of FIG. 1. Mixing circuits 11, 13 and 15; an intermediate frequency filter 12 having a band width of 9.0115 MHz.+-.4 KHz; an intermediate frequency filter 14 having a band width of 10.75 MHz.+-.1.5 KHz; an oscillator 17 that injects an output into mixing circuit 11; and an oscillator 18 that injects an output into mixing circuits 13 and 15 comprise the intermediate frequency stage of a triple conversion system. In other words, oscillator 18 that injects an output into mixing circuits 13 and 15 is used in common and is separated from a beat frequency oscillator 19. Thus, the interference waves are eliminated by changing the output frequency of oscillator 18 making use of the skirt characteristic of intermediate frequency filter 14.
The circuit shown in FIG. 3 effects the principle illustrated with respect to FIG. 1 with an intermediate frequency stage of a single conversion system. A mixing circuit 21 and an intermediate frequency filter 22 comprise the single conversion intermediate frequency stage. The local oscillator circuit serving mixing circuit 21 is a phase-locked loop comprising a voltage control oscillator 24, a low-pass filter 25, a phase comparator 26, a mixing circuit 28, and an oscillator 29. A mixing circuit 30 is inserted into this phase-locked loop to use the local oscillator 31 serving the mixing circuit 30 in common with the beat frequency oscillator for detecting circuit 23 and to change the output frequency of the local oscillator 31. The interference waves are eliminated by shifting them outside the band of the intermediate frequency filter 22.
However, in the case of receivers having intermediate frequency shifting circuitry as shown in FIGS. 1 through 3 above, the circuitry is effective in eliminating an interference signal on only one side of the desired signal--that is, a frequency either higher or lower than the desired signal. However, when interference signals exist on both sides of the desired signal--that is, at frequencies higher and lower than the desired signal, the circuitry has a shortcoming in that it cannot eliminate both interference signals.
In addition, when the circuit of FIG. 1 is used, the interference signal which has been eliminated by notch filter 6 comes unhooked when the output frequency of oscillator 10 is changed and notch filter 6 has to be adjusted again.